The present invention relates to an improved diaphragm for loudspeakers, such as moving coil loudspeakers, and to a method for making this improved diaphragm.
Loudspeakers play a critical role in determining the fidelity of sound systems, and loudspeaker diaphragms play a critical role in the performance of loudspeakers. A wide variety of materials have been used in the past to construct loudspeaker diaphragms, including paper, polypropylene, various metals, treated paper, and mixtures of polypropylene and carbon fibers. The article entitled "Reinforced Olefin Polymer Diaphragm for Loudspeakers" (J. Audio Eng. Soc., Vol. 29, No. 11, pp. 808-813, Nov. 1981) describes several polypropylene/carbon fiber diaphragms. Each of these materials has advantages and disadvantages, but in general each represents a compromise among several desired diaphragm characteristics, including high stiffness, low mass, insensitivity to temperature and humidity variations, and low manufacturing cost.
Paper loudspeaker diaphragms have been in widespread use for a considerable time period. Such paper diaphragms provide advantages in terms of inexpensive manufacture and relatively good sensitivity in view of their relatively low mass. However, paper diaphragms can exhibit a relatively low stiffness which can adversely affect the frequency response of the diaphragm. In addition, paper diaphragms are generally moisture-sensitive, and the acoustical properties of paper diaphragms are typically affected by variations in humidity. Moreover, paper diaphragms can become brittle and crack over time, particularly when repeatedly cycled over extremes of temperature.
Polypropylene loudspeaker diaphragms have been described, for example, in U.S. Pat. No. 4,190,746. In general, such polypropylene loudspeaker diaphragms can be manufactured with good frequency response characteristics in view of the relative stiffness of polypropylene. However, polypropylene is a relatively expensive component material as compared with paper, and it can present difficulties in manufacture. In particular, adhesives which are suitable for use with paper diaphragms are often unsatisfactory for use with polypropylene diaphragms. In addition, polypropylene diaphragms, although relatively insensitive to moisture, can be adversely affected by relatively low temperatures. When polypropylene diaphragms are heated (as, for example, when positioned on the rear deck of an automobile) they may be distorted by elevated temperatures, and may tend to relax away from the manufactured shape. In some cases, polypropylene diaphragms have been made with increased thickness in order to allow the diaphragm to tolerate higher temperatures without distortion. However, such increased thickness will typically increase the mass of the diaphragm, thereby reducing its sensitivity.
Thus, both paper and polypropylene diaphragms exhibit undesirable characteristics which may cause problems in many applications.